AT-cut quartz crystal resonators have been commonly used in a variety of electronics as a source of oscillation. To obtain a high-frequency, the frequency of an AT-cut quartz crystal resonator is typically multiplied to a predetermined frequency by a phase-locked loop (PLL). To obtain a low noise signal as well as a high-frequency, a surface acoustic wave device may also be used as a direct source of oscillation.
AT-cut quartz crystal resonators, which can provide stable frequency properties, have been commonly used in many electronics as a source of oscillation. However, an AT-cut quartz crystal resonator needs high-precision processing technique, such as flattening technique and thinning technique, to be used in high-speed computers, telecommunications equipment, and other electronics as a source of a high-frequency of oscillation.
A surface acoustic wave, which includes a longitudinal wave or an SV wave generated on the surface of a piezoelectric substrate, has a frequency proportional to velocity and inversely proportional to a wavelength. The device utilizing this surface acoustic wave typically has an excitation-electrode on the front surface of the piezoelectric substrate cut at a predetermined cutting-angle. The excitation-electrode includes a plurality of electrode-fingers arranged in a comb-like form. Adjusting the film-thickness of the excitation-electrode and the pitch between each of the plurality of electrode-fingers allow the device to have a predetermined frequency of oscillation.
Patent document 1 discloses a piezoelectric device that utilizes a Lamb-wave mode in the surface acoustic wave generated on a rotated Y-cut quartz crystal substrate. The piezoelectric device includes a comb-like excitation-electrode on the front surface of the substrate and a thin-film configured to adjust a frequency on the rear surface of the substrate. The piezoelectric device has a same quadratic function temperature behavior as that of conventional ST-cut quartz crystal resonators
Patent documents 2 and 3 disclose resonators that oscillate a lamb-wave. Compared with thickness-shear vibrators, such as AT-cut quartz crystal resonators, the lamb-wave resonators have better frequency properties because they have a cubic function temperature behavior. However, the cutting angle of the quartz crystal substrate of this resonator is specified by a rotation angle of two axes. Such a manufacturing complexity may lead to problems, such as fluctuations in a frequency temperature behavior and manufacturing difficulties.
Patent document 4 discloses a high-frequency resonator that includes a rotated Y-cut quartz crystal substrate specified by Euler angles.
Resonators disclosed in Patent documents 2 to 4 has a structure that includes a comb-like excitation-electrode on the front surface of a piezoelectric substrate. Such a structure does not include any thin-films configured to adjust a frequency on the rear surface of the piezoelectric substrate.
Patent document 5 discloses a relationship between the metallization-ratio of an excitation-electrode and the film-thickness of the excitation-electrode.
A resonator disclosed in Patent document 5 mentioned above has a structure that includes an excitation-electrode on the front surface of a piezoelectric substrate. This structure does not include any thin-films configured to adjust a frequency on the rear surface of the piezoelectric substrate, either.
Patent document 6 describes a method for adjusting a frequency. The frequency is adjusted by trimming the thin-film of an electrode on the opposite side of a surface on which a comb-like electrode is disposed.
Patent document 7 discloses another method for adjusting a frequency. The frequency is adjusted by trimming a thin-film disposed on a surface on which an electrode is disposed.